When you start looking closely, you will notice that trees are very complex organisms! In this section we are going to look closely at the stems of trees - you might know them as trunks!

Tree stems are in charge of some very important functions. Stems support the tree, move nutrients and water where they are needed, store nutrients and water until they are needed, and hold the leaves/needles so they can soak up the sunlight.

Inside Stems

What Makes a Ring?

Because trees live longer than most plants, their stems (trunks) grow differently. Trees develop woody stems that contain two types of tissues:

primary tissues

The first tissues formed by a plant
Examples: primary xylem and primary phloem

secondary tissues

Tissues generated by the growth of the cambium.
Examples: secondary xylem and secondary phloem

Secondary tissues are added each year. These additions cause something known as secondary thickening in the stems. This thickening is what makes a woody stem as woody stem. It's part of what makes a tree a tree.

You can see evidence of stem (trunk) growth by looking at a cross-section of an older tree. Each ring shows one year's worth of growth. But, what causes the rings?

A young stem has bundles of vascular tissues (tissues that carry fluid). The bundles are made up of xylem, phloem, and cambium layers.

As the tree ages, the cambium layers between the bundles come together.

As the tree gets older, a vascular cylinder is formed. About a year after the vascular cylinder is formed, a tree's first ring will appear.

Remember, the cambium is a meristem - a place of growth. Each growing season, new xylem cells are produced. These cells eventually clog and no longer transport sap. They become heartwood. These are the cells that make up the rings you see.

Year after year, new layers of xylem tissue are added and year after year a new ring is produced.

Reading Rings

Dendrochronologists are scientists who study the rings of trees. They use annual rings to date events in history - both in the history of the tree and of the natural world.

The annual rings of a tree can give scientists a snapshot the environment. You might be surprised by how much can be told by looking at the rings of a tree. Scientists have identified:

times of too much water (floods),

times of too little water (drought),

lightening strikes,

earthquakes,

insect infestations,

climate change.

Here's a very basic look at how it works. The thickness of the yearly ring, depends on the environment of the tree. Because the rings are created in due to addition of cells and tissues in the cambium, you can begin to make guesses about the environment by looking at the thickness of the rings.

Some years, rings will be thick. From this, we might hypothesize that the conditions for growing were excellent. Other years, the rings will be thin. This would signal that, for some reason, the growing conditions were not as good. Perhaps there was a drought or a disease.

The rings are just part of the story. Dendrochronologists use many scientific methods to date a tree. However, the annual rings of a REAL TREE allow them to become storytellers in a sense. They can identify patterns of growth over time that help scientists and agronomists better understand the climate and environment in which REAL TREES grow and live.

Bole is another name for a tree trunk!

bark:
the tough outer covering of the woody stems, branches, and roots of plants

cambium:
a layer of tissue between the inner bark and wood of a tree that produces new bark and wood cells; forms annual rings in trees

cork:
water resistant protective tissue formed outside of the cork cambium in woody stems

heartwood:
older, non-living wood in the center of a woody plant; usually harder darker in color than younger sapwood

phloem:
tissues that carry dissolved food through a plant

pith:
the soft, spongy tissue in the center of dicot stems

primary tissue:
the first tissues formed by a plant

sapwood:
outer wood formed in the cambium of a tree trunk; active in transport of water; usually lighter in color than hardwood

secondary tissue:
tissues developed by the growth of the cambium

shoot:
young growth germinating from a seed; sprout

stem:
the supporting structure of a plant

vascular:having vessels that carry fluids like blood or sap

vascular tissue:
tissues that carry water and nutrients through a plant

xylem:
a woody tissue in plants that helps support the plant and carries water and nutrients

Early Native Americans used the inner bark of the white pine as food. Later, the colonists used the inner bark as an ingredient in cough medicine.

Check out these sites for more information on tree rings and dendrochronology.